The present invention relates to a method and to apparatus for depositing thin layers.
The technical field of the invention is that of manufacturing thin-layer optical devices.
The invention applies in particular to fabricating optical filters essentially constituted by a stack of thin layers deposited in succession on a plane substrate.
Such fabrication conventionally makes use of a technique of depositing a material that is to constitute each layer on the substrate or on the stack inside an enclosure in which pressure is maintained at a low value (generally referred to as a xe2x80x9cvacuumxe2x80x9d enclosure), with the material being deposited by being xe2x80x9cevaporatedxe2x80x9d or xe2x80x9csublimedxe2x80x9d from a xe2x80x9csourcexe2x80x9d of said material followed by xe2x80x9ccondensationxe2x80x9d of the same material on a xe2x80x9ctargetxe2x80x9d which is constituted by the substrate or by the stack that is being built up; for this purpose, use is made of one or more electron or ion guns.
In order to obtain determined optical performance for the thin layer device, it is necessary to control the thickness of each deposited layer with sufficient accuracy to ensure that thickness error is minimized, and in particular is less than 1%.
For this purpose, various methods have been developed for monitoring the thickness of a layer while it is being deposited.
A xe2x80x9cmechanicalxe2x80x9d method consists, prior to starting the deposition process, in placing a resonant structure such as a quartz crystal inside the vacuum enclosure close to a substrate that is to receive the thin layers; the xe2x80x9cevaporatedxe2x80x9d material is then deposited on said structure as the deposition process advances in a manner that is assumed to be identical to the deposition of the material on the substrate; the deposit on the structure leads to a change in its resonant frequencies; by measuring at least one of these resonant frequencies it is possible to obtain an (indirect) indication concerning the thickness of the layer deposited on the resonant structure, and this thickness is assumed to be identical to the thickness deposited on the substrate; that indirect method of monitoring the thickness of thin layers being deposited does not enable the desired accuracy to be obtained.
There also exist xe2x80x9copticalxe2x80x9d methods of monitoring the thickness of thin layers that are being deposited, which methods are generally based on measuring the variation over time in the transmission or the reflection of a light beam directed on the stack while it is being built; such optical monitoring techniques include those in which the light beam is monochromatic, those in which the light beam is bichromatic, those in which the light beam is multichromatic, and those in which the light beam is xe2x80x9cbroad-bandxe2x80x9d.
Techniques using one or two wavelengths are particularly suitable for monitoring optical thicknesses of quarterwave stacks; conversely, the technique using a broad-band beam is well adapted to stacks in which the optical thicknesses are not quarter wavelengths; in the monochromatic technique, a transmission extremum is sought, generally by computing the derivative with respect to time of transmission, and an order to stop depositing material is issued when the derivative becomes zero; it is also possible to cause deposition to stop when the partial derivative of the transmission factor relative to the optical thickness becomes zero.
In a bichromatic technique, deposition is generally stopped as a function of the measured value of the difference in transmission at the two wavelengths under consideration.
In a broad-band technique, deposition is generally servo-controlled as a function of the difference between a predetermined transmission spectrum and a transmission spectrum as measured during deposition.
In a multichromatic technique, prior to performing deposition, a plurality of wavelengths are determined such that for each of them the transmission of a layer of the stack presents an extremum when the desired optical thickness is reached.
In practice, the thickness deposited on a reference substrate is monitored, and when a plurality of substrates are being deposited on a common support that is set into rotation, the reference substrate is at a distance from said substrates; as a result the accuracy of the monitoring is not sufficient.
U.S. Pat. No. 5,425,964 describes a broad-band optical monitoring method.
U.S. Pat. No. 4,311,725 describes a method and apparatus for measuring and controlling the deposition of a thin layer by combining a mechanical method and an optical or an electrical method: deposition is monitored on the basis of the ratio between a signal delivered by a crystal which is subjected to deposition and a signal representative of the transmittance, the reflectance, or the resistivity of the thin layer being deposited.
Although the known methods and apparatuses for monitoring the deposition of a thin layer can, in certain applications, present performance that is sufficient, other applications, and in particular the manufacture of optical bandpass filters of multilayer structure for processing telecommunications signals, require better performance.
An object of the invention is thus to provide an improved method and apparatus for monitoring vacuum deposition of thin layers.
In one aspect of the invention, an optical property such as transmittance or reflectance is measured on a plurality of thin layers (TL) or a plurality of stacks of thin layers which are being formed by vacuum deposition onto a plurality of substrates (or substrate portions), with measurements being performed by a plurality of light beams, each of said light beams being directed towards a respective one of said substrates; this makes it possible to determine said optical property for each of said TLs or for each of said TL stacks, and to control changes in and/or to interrupt deposition selectively for each of said TLs or said TL stacks.
By substantially simultaneous use of a plurality of monitoring light beams respectively associated with the thin layers being deposited on a plurality of substrates, it is possible to measure and control deposition individually on a plurality of substrates, thus making it possible to obtain improved uniformity in the deposits made in this way.
In a preferred embodiment, the monitoring light beams are obtained by splitting a single xe2x80x9cprimaryxe2x80x9d light beam so that the monitoring beams present characteristics which are homogeneous if not identical, thereby simplifying beam processing (after transmission or reflection by the devices being fabricated.
The single primary beam can be split into a plurality of monitoring beams by time division ; this can be achieved quite simply by moving the substrates inside the vacuum enclosure during the deposition process so that each of the substrates (stacks) is brought in succession onto the path of said single beam; in this way, the substrates or stacks to be monitored form a shutter because they are in motion, and the reflected or transmitted beam forms in succession as many monitoring beams as there are substrates (or stacks) that have passed through it.
The substrates or stacks preferably move in substantially continuous manner around a closed outline of circular shape so that all of the substrates are maintained at a substantially constant distance from the source, thereby encouraging continuous and uniform deposition of the material from which the thin layer is being formed; for this purpose, the substrates can be mounted on a support in the form of a circular ring that is set into uniform rotation about its axis of symmetry, throughout the duration of deposition.
In another aspect of the invention, in a method of fabricating a batch comprising a plurality of thin layer optical components by vacuum deposition, in which all of the components are placed on a common support that is mounted to rotate inside the vacuum enclosure, the substrates are placed on the support in such a manner as to ensure that they are situated at substantially the same height (or distance) from the source of material to be deposited, and during deposition, said optical property of at least one of said substrates is measured; by means of this method, the measurement performed is representative of how deposition is progressing on all of the substrates in the batch, and the accuracy of monitoring is better than when monitoring is performed on a reference that is remote from the substrates that are being coated.
To this end, in a preferred embodiment, the common rotary support is fitted with a hollow shaft within which the primary beam can propagate, and also with a member for deflecting the primary beam, such as a periscope secured to the support, so as to deflect the primary beam and direct it in such a manner as to cause it to pass through one of the substrates placed on the support.
The invention makes it possible to remedy the disparities observed between the optical performances of the various thin film devices in a single batch (being fabricated simultaneously) and when thickness monitoring is performed by taking measurements on a single reference device.
The primary beam is preferably caused to intercept each substrate or stack at a constant frequency which is high enough to ensure several hundreds, thousands, or tens of thousands of such interceptions while a single thin layer is being deposited; in particular, 900 to 90,000 interceptions are involved for each thin layer.
Thus, for each stack, several hundreds or thousands of measurements of said optical property are performed, thereby making it possible to monitor the optical thickness of the deposited layer with accuracy that is better than 1%.
Said time division of the primary beam to form a plurality of monitoring beams can also be obtained by causing said primary beam to perform a scanning movement over said stacks; under such circumstances, the stacks can themselves be mounted so as to be stationary or so as to move within the enclosure.
In a variant embodiment, the monitoring beams can be formed by frequency division of the primary beam; to this end, it is possible in particular to place a prism on the path of the primary beam so as to form a plurality of mono- or polychromatic beams on the basis of a single primary polychromatic beam.
In another variant embodiment, space division is performed on the primary beam, e.g. by using semireflecting plates.
The various means for splitting the primary beam into a plurality of monitoring beams can be combined with one another; they can also be used to split a plurality of primary beams.
In a method of the invention, it is possible to use said mono-, bi-, multichromatic or broad-band optical techniques; nevertheless, it is preferable to use a mono- or bichromatic technique when fabricating narrow band wavelength division multiplex (WDM) filters, the multichromatic technique when fabricating antireflection filters, and the broad-band technique when fabricating components that are required to present a particular spectrum.
In order to simplify the opto-mechanical implementation of the manufacturing apparatus, the beam splitter is preferably mounted inside the vacuum enclosure.
According to another aspect of the invention, means are also provided and used to selectively modify the rate of deposition on the various substrates or stacks forming a batch and processed simultaneously in the enclosure.
For this purpose, it is preferable to mount the substrates or stacks on a common support, in particular a support in the form of a ring mounted to rotate inside the enclosure in such a manner as to ensure that the substrates are disposed relative to the source in a zone where the intensity of deposition is identical (homogeneous); various means can be used to selectively modify the rate of deposition on the various substrates fixed on the common support: in a first embodiment, a screen is provided for each substrate and means are provided for selectively actuating each screen as a function of a signal or a data item representative of said optical property measured by the monitoring beam associated with said substrate or stack, thereby causing deposition to be stopped selectively.
In another embodiment, the means for individually modifying deposition rate on each of the substrates being treated simultaneously comprise means for moving each substrate relative to the source of material to be deposited; it has been found that moving a stack through a small amplitude inside the enclosure can enable the rate of deposition on said stack to be modified significantly; in particular, this small amplitude displacement can serve to modify the direction, or most preferably the distance of the target relative to the source, or indeed relative to a deposition activator device, such as an ion gun or an electron gun.
Such individual displacement of the substrate or stack can be obtained by making provision for each substrate to be fixed on a shifting or steering device which is itself fixed on the support which is common to the substrates; said shifting device can be in the form of a piezoelectric spacer interposed between the substrate and the common support; in a variant, each substrate can be fixed to the common support via a mechanical structure enabling the substrate to be placed in at least two different stable positions; in particular, it can comprise a tilting or cam system suitable for actuation by a lever (or stud).
In order to control the substrate displacement means individually so as to modify the rate of deposition on the substrate, means are provided for carrying at least one control signal issued by an electronic device as a function of data (or signal) processing performed on the measurement signal representing said optical property.
Given that the electronic device for monitoring and controlling deposition is situated outside the vacuum enclosure, and that the means for displacing each substrate individually are placed inside the enclosure, the signals for actuating the substrate displacement means or the screen activation means must pass through the wall of the enclosure; this can be done without contact, e.g. by a magnetic effect; alternatively, the signals can be conveyed by electrical conductors; under such circumstances, and when the common support is mounted to rotate inside the enclosure, it is possible to provide a rotary contactor device.
These means can be used in combination with the screens associated with each of the substrates so as to continue deposition selectively on substrates which are not shielded by their respective screens while stopping deposition on those substrates which are shielded by the presence of their screens.
Once evaporation and deposition have been stopped on all of the substrates, it is also possible to reduce selectively the thickness that has already been deposited on one of the substrates by deactivating its screen in order to make the substrate accessible to radiation from an ion gun for sputtering, while leaving the other substrates shielded by their respective screens.
Because deposition on the substrates within a single batch is controlled selectively and individually, it is possible simultaneously to manufacture components having optical characteristics that are different, said characteristics being recorded prior to fabrication in a memory associated with the electronic deposition control device; this makes it possible to manufacture small numbers of components at reduced cost.